The spontaneously hypertensive rat (SHR) develops sustained arterial hypertension at a young age, progressive cardiomegaly, and has a high incidence of cardiovascular deaths at an age younger than its non-hypertensive control strain. It is thus an attractive model for the study of the interaction of aging and "disease." The results presented here show that: (1) intra-arterial blood pressure in the SHR is elevated at 5 months compared to controls and this is sustained until 21 months; (2) cardiomegaly is 110 percent at 5 months and 143 percent at 21 months; (3) isolated muscle at 29 degrees C shows an interaction of age and "disease" for several parameters: (a) contraction duration; (b) developed force at short muscle lengths or in response to changes in the pattern of stimulation, and (c) recovery from hypoxia. Thus there are several important interactions of age and "disease" which are manifest in the performance of isolated cardiac muscle.